LONDON, June 17 (Reuters)- The deadly MERS virus remains a serious public health problem, especially with the approach of haj pilgrimages, but a recent surge in Saudi cases of the respiratory disease appears to be abating, the World Health Organisation said on Tuesday.

The Middle East Respiratory Syndrome (MERS) virus, which causes coughing, fever and sometimes fatal pneumonia, has been reported in more than 800 patients, mainly in Saudi Arabia.

It has spread to neighbouring countries and, in a few cases, to Europe, Asia and the United States. At least 315 people worldwide have died from the disease.

In a statement issued after the 6th meeting of its MERS emergency committee, the WHO said a surge in cases in Saudi Arabia that began in April has now decreased and "there is no evidence of sustained human-to-human transmission in communities".

"There have been significant efforts made to strengthen infection prevention and control measures," it said. As a result, "the committee unanimously concluded that the conditions for a Public Health Emergency of International Concern (PHEIC) have not yet been met".

Global health regulations define a PHEIC as an extraordinary event that poses a risk to other WHO member states through the international spread of disease, and which may require a coordinated international response.

The WHO stressed, however, that the MERS situation continued to be "of concern", especially given an anticipated increase in travel to Saudi Arabia related to the pilgrimages and religious festivals of Umra, Ramadan and the haj.

The WHO's assistant director general for health security, Keiji Fukuda, said the committee had urged vulnerable countries, especially those in Africa, to take concrete action ahead of Umra, Ramadan and haj with basic public health measures such as conducting surveillance for MERS, raising awareness about it and implementing basic infection prevention and control measures.

Millions of people travel to Mecca each year for the haj, the pilgrimage which all Muslims must perform at least once in their lifetime, if they are able. This year's will take place in October.

UNFORTUNATE AND TRAGIC

David Heyman, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine and one of the most prominent critics of Riyadh's handling of the MERS outbreak, said it was "unfortunate and tragic" that people are still getting infected, getting sick and dying form the virus almost two full years after it was first identified in humans.

"Case control studies, where risk factors for transmission of this virus from nature to humans, are necessary and could help determine the way or ways in which people are becoming infected, and this information could be used to stop primary infections," he said.

Fukuda said such studies were now in the process of being carried out in Saudi Arabia, and welcomed fresh efforts by authorities there to get on top of the outbreak.

MERS has been linked to camels, which many scientists think may be a major source of infection in people. Hundreds of new MERS infections were contracted by patients and health workers in hospitals in Saudi Arabia during recent months.

The Kingdom was previously criticised by the WHO and by public health experts for failing to implement basic hygiene and infection control measures in hospitals - allowing the virus to spread in clusters of health workers - and for failing to accurately track and report MERS cases.

International concerns over Saudi handling of the outbreak grew earlier this month when it said it had under-reported cases by a fifth.

But Fukuda said he was now confident Saudi authorities were taking MERS very seriously and working hard to control it.

"The Saudi government has made an extensive effort really to catch up on all the numbers and to provide them as quickly as possible," he told reporters on a teleconference from the WHO's Geneva. "I see a big amount of improvement taking place". (Reporting by Kate Kelland, Editing by Ralph Boulton)